This section defines some of the terms used in the description and claims. Whenever a reference is made to these terms, they should be construed in accordance with the following paragraphs.
Types of Polyethylene
Ethylene based polymers are available in many forms including highly branched low density polyethylene (LDPE) and more linear polyethylenes. Each form can make a distinct contribution towards processability and ultimate film properties. LDPE and linear polyethylene are regarded as distinct by people skilled in the art.
To obtain LDPE, ethylene can be polymerized using free-radical initiators under high pressure conditions. The free radicals trigger the incorporation of chain lengths along the length of a main chain so forming long chain branches, usually by what is known as a back-biting mechanism. The branches vary in length and configuration. LDPE can be described as heterogeneously branched. The polymer chains formed differ significantly and the molecular weight distribution as determined by GPC is broad. The average molecular weight can be controlled with a variety of telogens or transfer agents which may incorporate at the chain ends or along the chain. Comonomers may be used such as olefins other than ethylene or minor amounts of olefinically copolymerizable monomers containing polar moieties such a carbonyl group.
LDPE is defined in the specification and claims as a polymer comprising at least 85 mol % of units derived from ethylene which is heterogeneously branched and contains less than 7.5 mol % of units derived from comonomers containing polar moieties such a carbonyl group, including ethylenically unsaturated esters, e.g. vinyl acetate, ethylene methyl acrylate, ethylene methacrylic acid or ethylene acrylic acid.
Other types of ethylene based polymers that are not included in the above LDPE definition include heterogeneously branched ethylene vinyl acetate containing more than 7.5 mol % of comonomer having polar groups.
To obtain the more linear ethylene based polymers, referred to herein as linear polyethylene, catalytic polymerization mechanisms are used. Polymerization may be performed with Ziegler-Natta catalysts comprising generally a transition metal component and in most cases an activator or cocatalyst. Monomers such as ethylene or other olefin comonomers incorporate principally at the end of the polymer chain. Backbiting mechanisms are substantially absent. The molecular weight distribution as measured by GPC Mw/Mn is relatively narrow. Such polymers tend to be more linear and have no or low levels of long chain branches. As used herein in the description and claims, references to non-branched linear polyethylene refer to polymers having an I21.6 linear pe/I2.16 linear pe ratio of less than 30.
If long chain branches are present in measurable amounts, their length and structure is assumed to be similar and linear. They may be referred to as homogeneously branched linear polyethylene. This term as used herein in the description and claims refers to polymers having an I21.6 linear pe/I2.16 linear pe ratio of greater than 35. The molecular weight distribution is narrow relative that that prevalent for LDPE. Because of the sensitivity of the catalysts to poisoning by polar groups, monomers having polar groups cannot be used. The main comonomers are alpha-olefins.
Linear polyethylene is defined in the specification and claims as a polymer comprising at least 65 mol % of ethylene derived units and a balance of units derived from an alpha-olefin having from 3 to 12 carbon atoms which is not branched or, if branched, is homogeneously branched. Generally these polymers have an Mw/Mn as determined by GPC DRI as described herein of less than 4.
Linear polyethylene may be sub-divided into different types depending on their density. The main groups are very low density polyethylene (VLDPE), linear low density polyethylene (LLDPE) and high density polyethylene (HDPE). In the general literature the stated density ranges for these polymers may vary. In the specification and claims, VLDPE is defined as a linear polyethylene having a density of less than 0.91 g/cm3; LLDPE is defined as a linear polyethylene having a density of from 0.91 up to 0.94 g/cm3; and HDPE is defined as a linear polymer having a density of above 0.94 g/cm3.
Linear polyethylene may also be subdivided having regard to the nature of the catalysts system used which influences homogeneity and so the overall properties in processing and properties of the film produced. The prefix “zn” is used in the specification and claims, as in “znLLDPE”, to indicate that the catalyst system used titanium as the transition metal component and an aluminum alkyl as cocatalyst. The prefix “m” is used in the specification and claims, as in mLLDPE, to indicate that the transition metal component used was a single site catalyst, which generally refers to a metallocene activated by methods well known for such components, such alumoxane or a non-coordinating anion. “zn” linear polyethylene types tend to have a greater heterogeneity in terms of molecular weight distribution and composition distribution as compared to “m” linear polyethylene types, as may be determined by suitable fractionation techniques appropriate to the density concerned, such as a measurement of the compositional distribution breadth index (CDBI) or a Crystaf measurement.
As used herein in the description and claims “zn” linear polyethylene types refer to polyethylenes, analyzable by elution fractionation, having a CDBI of less than 45% and “m” linear polyethylene types refer to polyethylene having a CDBI of greater than 50%, the CDBI being determined as described in WO93/03093 (U.S. Pat. No. 5,206,075). At low densities other fractionation techniques can be used to separate “zn” and “m” types of linear polyethylene.
Catalytic polymerization mechanisms are also used to produce linear polymers based on other olefins, mostly propylene. Examples include propylene based polymers such as polypropylene homopolymer, random propylene copolymer (RCP) as well as propylene based elastomers (PBE), including those described in WO99/07788 and WO2003/040201 having varying degrees of randomness or blockiness. The term “other linear polyolefin polymers” is used in the specification and claims to refer to other linear polymers generally using a catalytic polymerization mechanism with units derived from one or more olefin monomers, that may or may not be branched, but which exclude linear polyethylene as defined above.
In describing the compositions in the description and claims all percentages by weight are based on the total weight of polymer in the compositions, excluding any other non-polymeric additives, unless otherwise mentioned.
Coextrusion Processes
Films can be extruded by cast extrusion or blown film extrusion. The invention is concerned with blown film extrusion and especially coextrusion. The term coextrusion in the specification and claims refers to an extrusion process where at least two molten polymer compositions are extruded and bonded together in a molten condition in the die exit. Films are formed, while cooling progressively, after a complex interplay of stretching, orientation and crystallization until the film reaches a take up device enclosing the top of the bubble, such as a pair of pinch rollers.
In blown film extrusion the film is pulled upwards by for example pinch rollers after exiting from the die and is simultaneously inflated and stretched transversely sideways to an extent that can be quantified by the blow up ratio (BUR). The inflation provides the transverse direction (TD) stretch, while the upwards pull by the pinch rollers provides a machine direction (MD) stretch. As the polymer cools after exiting the die and inflation, it crystallizes and a point is reached where crystallization in the film is sufficient to prevent further MD or TD orientation. The location at which further MD or TD orientation stops is generally referred to as the “frost line” because of the development of haze at that location.
Variables in this process that determine the ultimate film properties include the die gap, the BUR and TD stretch, the take up speed and MD stretch and the frost line height. Certain defects tend to limit production speed and are largely determined by the polymer rheology including the shear sensitivity which determines the maximum output and the melt tension which limits the bubble stability, BUR and take up speed.
Discussion of Prior Art References:
U.S. Pat. No. 4,518,654A published 21 May 1985 discusses the use of blends of LDPE and linear polyethylene in mono-layer stretch wrap films. The LDPE used in the examples has an MI of 2.0; the deformation rate is not indicated.
U.S. Pat. No. 5,922,441A published 13 Jul. 1999 produces a multi-layer stretch film using a core layer containing LDPE with an MI of 1.9 and 3.3. The deformation rate and haze is not indicated.
US2004/0048019A1 published 11 Mar. 2004 discloses coextruded stretch films (see [0193]) using a blown film process and mLLDPE's. No examples of compositions that are especially selected for use in combination in multi-layer films are disclosed.
US2006/0188678 published 24 Aug. 2006 discloses multi-layer coextruded film made by blown extrusion for use in heavy duty packaging. Clarity is not a major requirement for such use. Table 3 Example 2 discloses a multi-layer film with the core containing 5 wt % of Escorene LD150BW which has an MI of 0.75 and a skin layer containing 9 wt % of the same LD150BW; almost double that in the core layer. Haze is not reported.
WO2004/011456 published 5 Feb. 2004 discloses (see page 7 line 21 onwards) multi-layer shrink films that may be coextruded including blown film extrusion. Examples 2 and 3 disclose the combination of multi-layer films, some of which have an LLDPE in the core and LDPE in the skin. Haze is not reported.
Example 23 in EP1529633 published 11 May 2005 discloses a multi-layer blown coextruded film having a core layer comprising 80 wt % Escorene LD514BA and 20 wt % of an HDPE and two skin layers in contact with the core layer of 95 wt % of an mLLDPE and 5 wt % of the LD514BA. The MI of the LDPE is not indicated. Its replacement by the HDPE is said to lead to reduction in haze.
WO2005/014762 published 17 Feb. 2005 describes multi-layer film for stretch hood application. The examples show skin layers of mLLDPE and a core layer of heterogeneously branched EVA containing less than 2.3 mol % of units derived from vinyl acetate (see Table 1). Information provided is insufficient to determine the frost line height (FLH) and diameter at the FLH location and does not disclose the deformation rate.
WO2004/022646 published 18 Mar. 2004 describes shrink films including multilayer films (see [0010] and [0095]). LDPE for the core layer may have a broad range of properties covering all commercially available LDPE's. Examples 18-21 disclose blown coextruded multi-layer films using core and skin layer of the same composition with an LDPE-D having an MI of from 0.75 and a density of 0.923 (See Table 5) blended Resin B and C being mLLDPE's. The haze is in excess of 10 (see Table 10).
U.S. Pat. No. 5,248,547 dated 28 Sep. 1993 and related cases U.S. Pat. No. 5,261,536 and U.S. Pat. No. 5,431,284 disclose a three layer film with an LDPE core having a melt index of 1 to 25 in the broadest range and linear PE skin layers in the form of znLLDPE having a melt index range of from 1 to 10. The skin layers may contain LDPE (see column 6). In the examples the znLLDPE has an MI of 3.3 and the core layers comprise LDPE having an MI of 1.9 or 7. No indication is provided of the deformation rate or haze.
U.S. Pat. No. 6,521,338 dated 18 Feb. 2003 has cling layer using a homogeneously branched linear polyethylene and an LDPE core layer in an A/B multi-layer structure. The LDPE has a melt index in excess of 1 g/10 min. In the examples the LDPE's have a melt index of 1.9 and 3.27 g/10 min. Haze and extrusion conditions are not detailed.
U.S. Pat. No. 6,482,532 published 19 November in Example 2 a film having a low haze of 1.1 in Table 2. The film is a multi-layer film with homogeneously branched low density linear polyethylene skin layers having a density of 0.902 and a melt index of 3.0 g/10 min and a core layer of an LDPE having an MI of 5.0 g/10 min.
U.S. Pat. No. 6,368,545 dated 9 Apr. 2002 seeks to improve the clarity of blown coextruded films wherein the melt extrusion temperature and/or the density of a core layer is higher than the equivalent for the skin layer or layers. The skin layers are formed from a composition consisting of single znLLDPE or mLLDPE polymer. The core layer may be formed of an LLDPE, optionally admixed with an HDPE or from blends with varying amounts of an LDPE (LD157 CW having an MI of 0.6 and a density of 0.932 g/cm3) and an mLLDPE. There is no description of the conditions under which the multi-layer films were coextruded. While the use of compatible LLDPE's in the skin and core layers gives rise to haze values less than 8, examples containing LDPE have haze levels in excess of 8.
Column 8 lines 42-48 discusses the benefit of using grades with higher shear sensitivity for the core layer so as to increase the melt strength to sustain the bubble formed after extrusion by the molten extrudate but there is no suggestion that deformation rates may exceed the conventional levels for the type of multi-layer film concerned. In keeping with conventional thinking for avoiding surface irregularities, the use of LDPE in the skin layer was also was contemplated although not exemplified.
WO2006021081A discusses blown film extrusion processes and various factors influencing the process outcome stating that “the use of a small amount of HPLD which has a high molecular weight has been observed to allow large production increases when producing film from homogeneously catalyzed lldpe”. But there is no indication as to how such a blend would be accommodated in blown film coextrusion.
Problems and Objectives
The film properties are the result of the combined effect of the coextrusion process conditions and the combination of polymer compositions selected for the different layers. In spite of the various suggestions for improvement in references cited above, it remains difficult to achieve good optical properties in combination with high production speeds and film performance, especially when narrow composition and molecular weight distribution linear polyethylenes, such as for example mLLDPE, are used to improve end use film performance.
It has been difficult to reach low haze value of less than 10 and especially less than 5. Often use of LDPE in the skin layer has been deemed necessary. Skin layers substantially consisting of mLLDPE's, useful in providing heat seal characteristics (see WO93/03093) remained difficult to process absent addition of other components. While high deformation rates were used in the extrusion of thin film structures, more substantial films were generally processed at lower speeds to reduce surface defects.
Good optical appearance as expressed by low haze values of less than 8 or 4 have thus been difficult to achieve and even more difficult to combine with higher process speeds.
Ethylene based polymers are generally seen as appreciably inferior from an opticals point of view when compared to oriented polypropylene films. Furthermore for stretch and cling film applications, additives other than polymers have to be added in significant amounts such as liquid polyisobutylene (PIB) or special polymers such as lower density VLDPE to induce enough cling.
It is among the objects of the invention to facilitate the production of low haze films that may be used for various applications, and optionally facilitate production under conditions that permit faster line speeds and/or reduced haze levels by a judicious combination of polymer component selection and processing conditions.
Apart from optical properties films also require good seal strength so as to permit the reliable packaging on a continuous packaging line at high speed. Apart from the above, control of stretch, shrink and/or heat sealing properties may also be desirable. It is among the objects of the invention to improve film coextrusion in relation to one or more of the above aspects.